This may shock you, but I don't think I'm mentioning any new ideas here:
Four adults don't want to travel in a Yaris, Fit, or Versa. Maybe on short trips, but that's it.
The average US family consists of four people. They carry things.
Couples go out together. (See above.)
People buy stuff and have to fit it inside the vehicle.
The gas tank range stinks on compact cars.
Good luck easily fitting and removing two full-size pieces of luggage in many sub-compacts.
Average United Statesian doesn't pay much more for entry-level mid-sized car, and licensing, insurance, or registration taxes/fees aren't much greater (if at all). Probably sees larger car as prestigious, more comfortable, and safer, doesn't care as much about being more nimble or having better fuel economy.
Now, a couple or single person doesn't really
need that much vehicle (half of the country could literally get by with a subcompact) but maybe their ubiquity and sales figures tells you that the mid-size it's just about right for many.
...Yes, I still Drive this small car on weekends Disclaimer